Membrane filters are commonly used in domestic and industrial water treatment systems to separate solids from liquids. Not only are they used to treat wastewater but they are also used to improve the water quality of lakes, ponds and rivers. A typical wastewater treatment system includes the biological decomposition of the wastewater using microbes in a bioreactor followed by the filtration of the decomposed wastewater through a membrane filter. The resulting permeate that passes through the membrane filter is the treated water whilst the solid debris separated from the permeate collects on the membrane surfaces or as sludge in the bioreactor.
It will be appreciated that with use, a solid deposit commonly known as “filter cake” builds up on the membrane surface. The thickness of the filter cake increases with time of use. This membrane fouling decreases the flow of the wastewater through the membrane filter and gradually reduces the efficacy of the membrane filter until it must be replaced or cleaned. This can lead to down-time of the treatment process which is both costly and inconvenient and increases operating costs. Often an over-sized membrane filter must be utilized to account for the increasingly unusable membrane surface over time and to prolong the time before a change of the membrane filter is required.
Apart from removing the membrane filters to clean them, there are a number of known physical methods for cleaning in vivo. These include chemical cleaning of the membrane filters in situ; backwashing or back pulsing using a liquid permeate or a gas to apply pressure from inside the membrane filters to release solids on the membrane surface; and membrane surface scouring or scrubbing using a gas in the form of bubbles in a liquid.
Backwashing is effective mainly with tubular membranes, such as hollow fibre membranes, as the applied pressure deforms other types of membranes including most flat sheet membranes.
Membrane surface scouring can be used on all types of membranes so is more common. This method uses gas bubbles to flow over the membrane surface to release the caked solids back into suspension in the membrane separation reactor and settle to the bottom as sludge. The sludge can then be removed by various means such as draining. A problem with the scouring method is that gas bubbles take a path of least resistance across the membrane surface as they rise. This can leave a considerable area of the membrane surface untouched by the gas bubbles and the filter cake intact in these areas.
A number of patent applications, such as U.S. 20090223895 and U.S. 20090026139, describe plates or sleeves placed adjacent hollow fibre membrane surfaces to maintain the scouring gas bubbles against at least a portion of the membranes. However, the gas bubbles may still take the path of least resistance as they rise across the membrane surfaces thereby leaving parts of the membrane surfaces unscoured.
Ndinisa et at (Separation Science and Technology, 41, 1383-1409, 2006) placed baffles adjacent both sides of a flat sheet membrane in the riser section in a tank to attempt to correct the uneven distribution of bubbles scouring the membrane surface. The baffles comprised small rectangular channels 1 cm wide and 7 mm deep. The channels did not extend along the baffle in a continuous fashion but were interrupted by strips to avoid baffle sway by the air bubbles.
Therefore, it is desired to overcome or reduce at least some of the above-described problems.